Recently, ionomer resin has been widely used for cover material of golf balls. This is because the ionomer resin is superior in rebound characteristics, durability, processability and the like. However, since the ionomer resin has high rigidity and hardness, there are problems that in the resulting golf ball, shot feel is hard and poor; and spin performance is not sufficiently obtained, which degrades controllability, when using ionomer resin as the cover material.
In order to improve the problems, it is proposed to soften the cover by blending the hard ionomer resin having high rigidity with terpolymer-based soft ionomer resin. However, when good shot feel and spin performance are accomplished in case of the blend of the hard ionomer with the soft ionomer resin is used, rebound characteristics of the resulting golf ball are largely degraded. In addition, since the surface of the cover of the golf ball is easily abraded by grooves on a face surface of the golf club when hit by an iron club, it is problem that the surface of the golf ball becomes fluffy or begins to split finely, and scuff resistance (chunking resistance) is poor.
In order to solve the problems, it has been proposed to use thermosetting polyurethane composition or a blend of ionomer resin with polyethylene as a cover material of golf ball (Japanese Patent Kokai publication Nos. 102628/2000, 299965/2001 and the like).
In Japanese Patent Kokai publication No. 102628/2000, a cover material for golf ball comprising heat mixture of (a) ionomer resin, (b) polyethylene and (c) epoxy group modified polyolefin, as a main component, is disclosed. By using the cover material, dispersibility of the polyethylene in the ionomer resin is improved, which improves the wear resistance, but scuff resistance is not sufficiently obtained.
In Japanese Patent Kokai publication No. 299965/2001, a golf ball comprising a core, a boundary layer covering the core, a wound layer covering the boundary layer, and a cover covering the wound layer is disclosed. The boundary layer comprises thermoplastic material having a Shore D hardness of 40 to 85, and the cover comprises polyurethane material. The polyurethane layer is formed from cured article of a thermosetting polyurethane composition comprising p-phenylene diisocyanate. When the thermosetting polyurethane composition is used for the cover material, compared with the blend of the hard ionomer with the soft ionomer resin, the scuff resistance is excellent, but is not sufficiently obtained. In addition, since a process of covering a cover on a core is complicated, large-scale production is difficult, and the productivity is degraded.
In Japanese Patent Kokai publication Nos. 102628/2000 and 299965/2001, the cover is designed to have excellent scuff resistance in view of a formulation and wear of the cover material. However, a golf ball, of which the scuff resistance is not sufficiently obtained, is present even if it satisfies the factors. In Japanese Patent Kokai publication Nos. 102628/2000, the wear is reduced by using the above cover material in order to improve the scuff resistance. However, the low wear material is not always the material having excellent scuff resistance. When hit the golf ball by an iron club, grooves on a face surface of the iron club is cut into the surface of the golf ball (cover and paint layer) to bite off it, and the scuff phenomenon occurs. It is considered that the scuff phenomenon is different from the wear of the surface of the golf ball. The golf ball having excellent scuff resistance is not obtained by checking only the wear resistance as a evaluation item when developing the golf ball. Therefore, an index based on the scuff phenomenon is required when developing the golf ball having excellent scuff resistance.